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I've had a rattle in my passenger side tail pipe for months and have just lived with it until this week. The butterfly valve would rattle when it was in the closed position. I took it into a BMW dealer to be fixed this week and they said that they were directed by BMW to spot weld it in the open position.
From what I've read, it's purpose is to reduce the sound at low rpm's. If that's it's only purpose, I'm fine with the fix.(It sounds good) However if it is also for back pressure, I'd like them to actually fix the problem by replacing the muffler.
BMW obviously engineered the flap for a reason and I probably could demand that it be corrected under warranty.
If anyone knows the exact purpose of the valve, please let me know.

Dunno what might be in engineers' minds (I've worked with engineers a lot in my lifetime and that's a permanent bafflement) but can say no adverse effects of having the butterfly valve open all the time. A bunch of us have removed the short vacuum line that actuates it and capped both ends with no ill effect and better low-rpm sounds. I'd say fret not.

Dunno what might be in engineers' minds (I've worked with engineers a lot in my lifetime and that's a permanent bafflement) but can say no adverse effects of having the butterfly valve open all the time. A bunch of us have removed the short vacuum line that actuates it and capped both ends with no ill effect and better low-rpm sounds. I'd say fret not.

+1, I did the same thing on my R32. Car sounds better and I have observed no adverse effects. Has been in to the dealer for several routine maintenance visits and no mention has been made of any effect on engine diagnostic codes.

When I was a kid, we'd sometimes clip playing cards to our bicycles for a cool sound as it slapped the spokes. However, I don't remember the man at the bike shop doing the same to prevent me from hearing a rattle in the chain.

Dunno what might be in engineers' minds (I've worked with engineers a lot in my lifetime and that's a permanent bafflement) but can say no adverse effects of having the butterfly valve open all the time. A bunch of us have removed the short vacuum line that actuates it and capped both ends with no ill effect and better low-rpm sounds. I'd say fret not.

From the Society of Automotice Engineering and bafflement dept, the light vehicle sound standard is SAE J1492. The sound level is measured and averaged from idle to 3/4 of rated engine speed ramped up over a max 15 second period. J1492 details how to conduct the test based on exhaust orientation, location of instrumentation, surface, backgound noise, etc. 95dB (A Scale) is the max most states and mfgs have adopted. So if it is quieter at idle it can make more noise at the higher speeds.

Thus the flapper in on leg of the exhaust system, to keep the average sound pressure down at idle. The flapper has nothing to do with vehicle warm up.

From the Society of Automotice Engineering and bafflement dept, the light vehicle sound standard is SAE J1492. The sound level is measured and averaged from idle to 3/4 of rated engine speed ramped up over a max 15 second period. J1492 details how to conduct the test based on exhaust orientation, location of instrumentation, surface, backgound noise, etc. 95dB (A Scale) is the max most states and mfgs have adopted. So if it is quieter at idle it can make more noise at the higher speeds.

Thus the flapper in on leg of the exhaust system, to keep the average sound pressure down at idle. The flapper has nothing to do with vehicle warm up.

From the Society of Automotice Engineering and bafflement dept, the light vehicle sound standard is SAE J1492. The sound level is measured and averaged from idle to 3/4 of rated engine speed ramped up over a max 15 second period. J1492 details how to conduct the test based on exhaust orientation, location of instrumentation, surface, backgound noise, etc. 95dB (A Scale) is the max most states and mfgs have adopted. So if it is quieter at idle it can make more noise at the higher speeds.

Thus the flapper in on leg of the exhaust system, to keep the average sound pressure down at idle. The flapper has nothing to do with vehicle warm up.